Cosmological optical depth ($\tau (\eta )$)

The optical depth from conformal time $\eta$ to observation at ${\eta }_0$ isThe factor $e^{-\tau (\eta )}$ is the probability that a photon travels from $\eta$ to the observer without another scattering.

The visibility functionis the probability density for the conformal time of a CMB photon's last scattering.

The CMB line-of-sight solution separates sharply localized last-scattering sources, weighted by the cosmological visibility function, from integrated gravitational sources weighted by $e^{-\tau }$ along the photon trajectory.